There has been a great deal of interest in recent years in the use of bio-oxidation to recover metals from sulfide ores. In such ores, the sulfides trap, or occlude, the metal particles within sulfide minerals, such as iron pyrite for example. The bio-oxidation techniques use natural microorganisms to catalyze the oxidation of sulfides in the ore into soluble sulfates, in order to adequately expose the metal in the ore for subsequent extraction. Typical metals which may be recovered in this way include gold, silver, copper, zinc, nickel or cobalt.
In recent years, the gold industry has shown a particular interest in the use of bio-oxidation techniques for the recovery of gold, in large part because of the high value of gold. The primary goal of the gold mining industry is cost-effective recovery of gold from ore, and the most commonly used techniques for gold recovery from ore are smelting and cyanidation. However, a great deal of ore is to be found in ore which is naturally resistant to conventional recovery techniques. Such ore is called “refractory,” and usually contains gold particles which are locked, or occluded, within sulfide minerals. To obtain adequate gold recovery from such refractory ore, the ore must first be subjected to a pre-treatment process in which the sulfide minerals are degraded by oxidation. The ore may then be treated by a traditional reagent such as cyanide to dissolve the gold, in order to recover the gold from the treated ore.
Such bio-oxidation techniques are particularly useful for pre-treatment of mine tailings, which are the byproducts of mining operations. Not only does this allow gold extraction from highly refractory ores, but it provides the added benefit of removing a barrier to redevelopment and a potential environmental hazard. Bio-oxidation is particularly well suited to the pre-treatment of tailings, as the low gold concentrations found in such tailings are not a problem for the microorganisms involved. The microorganisms simply ignore the waste products in the ore, and proceed to oxidize the sulfides surrounding the gold, often resulting in ultimate extraction recoveries not achievable by other methods
One method of bio-oxidation used to pre-treat sulfide-refractory gold ores is heap bio-oxidation, which is described in U.S. Pat. No. 5,246,486 to Brierley et al. In this method, coarsely ground (P80>¼″)1 refractory ores are first agglomerated while being inoculated with a microorganism slurry, then heaped onto a leach pad with aeration and drain lines. This is referred to as a “free-drained” system; i.e., one in which there is no water table within the bed, no part of the bed is flooded, and the water leaves the system drained by gravity. Initially the inoculum is grown in a tank, but after the heap oxidation process matures, the solution draining from the heap contains the organisms and is used as the inoculum. The bio-oxidation continues until the predetermined target level of sulfide oxidation has been achieved. The ongoing sulfide oxidation levels are determined by the analysis of sulfate concentration in the bioreactor effluent solution and the bioreactor effluent cumulative mass flow. Once the bio-oxidation is complete, the ore is removed from the pad and lime is added to neutralize the ore. This makes the microorganisms in the ore become dormant, and also conditions the bio-oxidized ore for cyanide leaching to extract the gold. 1“P80” is a commonly used abbreviation in the mining industry, and means that 80% of the ore particles are finer than the specified size—in this case, ¼ inch.
Heap bio-oxidation can permit ultimate gold recoveries in the range of 60-70% from refractory ore. In addition, it uses inexpensive pond liners and allows air addition via high-volume blowers, which are relatively efficient and low-cost. However, heap bio-oxidation suffers from certain inefficiencies, primarily due to the large particle sizes, typically with a P80 of approximately ½ inch and no finer than a P80 of ¼ inch, with −150 [Tyler] mesh2 (106 microns) fines totaling less than 10% by weight of the total ore (expressed in the industry as P80=¼″, <10% −150 mesh). This large particle size causes “channeling,” in which water and solution seek large gaps between particles, and thus tend to flow by the ore without making substantial contact. To counter this channeling effect, a relatively high solution application rate is utilized in order to maintain contact with the ore. However, such high solution application rates result in a relatively thick layer of solution around each ore particle which impedes air flow through the heap, so that the oxidation rate is significantly slowed. 2A minus sign in front of a size designator, such as mesh or microns, followed by a percentage, is a standard abbreviation used in the mining industry to indicate that the specified percentage of the ore particles are finer than the specified size. In this case, the abbreviation is used to signify that less than 10% of the particles are smaller than 150 mesh (106 microns).
Further, it has long been known that the more finely ground an ore, the more efficiently it may be oxidized and the higher the ultimate gold recoveries would be in the subsequent gold recovery process. This is because as a given quantity of ore is ground into smaller particles, the overall surface area of that quantity of ore is increased. Since an increased surface area increases the contact with the oxidizing solutions, the oxidation proceeds at a faster rate, and is also more complete. However, a great deal of experience with heap leach gold cyanidation led to the conclusion that particle sizes less than P80=¼ inch tended to migrate through the heap, until ultimately they bind together into a clay-like mass, thereby “plugging” the flow of both solution and air through the heap. Since such plugging would render the heap bio-oxidation extremely inefficient, no use of particle sizes smaller than P 80=¼ inch has traditionally been attempted. Unfortunately, this perceived inability of heap bio-oxidation to utilize smaller particle sizes has greatly limited the efficiency and thoroughness of the oxidation achievable with the process. Notably, the typical 60-70% overall gold recovery could potentially be significantly higher if the oxidation were more thorough. In addition, the time for completing the heap bio-oxidation process is typically in the range of 180-360 days, thereby adding substantially to the heap bio-oxidation capital and operating costs. This heap retention time could also potentially be greatly reduced, if smaller particles could be accommodated by the heap bio-oxidation process.
One attempted solution to the above-mentioned problems with heap bio-oxidation has been agitated tank bio-oxidation. Agitated tank bio-oxidation is an alternative to heap bio-oxidation which allows for the utilization of much smaller particles (<100 microns). In this process, large quantities of oxygen and carbon dioxide are dissolved into a finely ground slurry of ore. Plugging problems which might otherwise be associated with such fine particles are avoided by utilizing a mechanically agitated tank to house the process. While such tanks are an effective way to allow very fine particles to be used in the process, they are highly expensive to purchase and to operate, and thus add greatly to the cost of the oxidation. Air addition into the agitated tank is also expensive and difficult to achieve, as the air must be added as extremely fine bubbles, and under sufficient pressure to overcome the pressure associated with the solution depth of the flooded tank. In addition to being costly, the air addition and the tank agitation render the whole process much more complex than traditional heap bio-oxidation. Ultimately, agitated tank bio-oxidation typically results in an accelerated retention time of 5-8 days, with an overall ultimate gold recovery of 85-90%.
There is thus a need for a pre-treatment bio-oxidation process for refractory gold ore which would allow significantly smaller particle distributions to be utilized, thereby greatly improving overall gold recovery and shortening bio-oxidation retention times. Ideally, the process would utilize a free-drained system, and thus would avoid the cost and complexity of agitated tank bio-oxidation.